Take a look at the full headers of the message being returned as a duplicate.  While I'm not a betting man, I'd be willing to wager that they'll show the mail went through some list recipient's mail system and looped back to LISTSERV.
Nathan
From: LISTSERV site administrators' forum [mailto:[log in to unmask]] On Behalf Of GT
Sent: Friday, January 18, 2013 8:24 AM
To: [log in to unmask]
Subject: Rejected Posting - Message Is Still Distributed

Hello,
There is a list that produces the following error message if a posting is Replied to, however, the message does get distributed:
Your message  is being returned to  you unprocessed because it  appears to
have already  been distributed  to the  CCR-NETWORK list.  That is,  a
message  with identical text  (but possibly with different  mail headers)
has been  posted to the list  recently, either by  you or  by someone else.
If you have  reason to resend this message to the list (for instance because
you have been notified of a hardware failure with loss of data),  please
alter the text of the message in some way  and resend  it to the  list.
Altering the  "Subject:" line  or adding blank lines at the top or bottom of
the message is not sufficient; instead, you should add a  sentence or two at
the top explaining why you  are resending the message. This explanation
will help the other subscribers  understand why they are getting two copies
of the same message.

I setup a test list configured identical to the list in question, and had no errors with multiple replies. I also ensure that the subscribers were also configure identically.
Here is the configuration of the list:
Notify= No
Default-Options= SUBJ
Default-Options= REPRO
Subject-Tag= "CCR-Listserv"
Attachments= Yes
Subscription= By_Owner,Confirm
Ack= Yes
Confidential= No
Validate= No
Reply-to= List,Respect
Review= Public
Send= Public,Confirm,Non-Member
Errors-To= Owner
Owner= [log in to unmask]<mailto:[log in to unmask]>
Notebook= No

Thanks!


________________________________

To unsubscribe from the LSTSRV-L list, click the following link:
http://peach.ease.lsoft.com/scripts/wa-PEACH.exe?SUBED1=LSTSRV-L&A=1

############################

To unsubscribe from the LSTSRV-L list:
write to: mailto:[log in to unmask]
or click the following link:
http://peach.ease.lsoft.com/scripts/wa-PEACH.exe?SUBED1=LSTSRV-L&A=1